1. Field of the Invention and Contract Statement
The invention relates to an improved drain valve for a furnace for the melting of thermoplastic material. The invention also relates to an improved drain valve for a furnace for glass melting that has a drain cavity for withdrawing molten glass from the furnace.
2. Discussion of Background and Prior Art
For normal operation of a slurry-fed melter, glass is poured into canisters through the pour spout by either continuous overflow, or by applying a differential pressure between the pour spout and melter plenum. Both pour techniques are acceptable methods of removing the glass from a melter. However, a problem is encountered when as much of the glass as possible must be removed from the melter when it has reached the end of its useful life or prematurely failed. Approximately three canisters of glass could probably be removed by the normal pour technique, however, one to two canisters would remain in the melter. The remaining glass would have to be removed by alternate methods.
The draining of the contents of glass melters (in a canyon environment) has always been a difficult matter, particularly with repeated draining. The basic requirements of a drain technique as applied to a glass melter are: (1) that the drain technique must be capable of draining the molten glass from the glass melter at any time at which solidification of the glass pool appears imminent due to serious equipment or process malfunction; and (2) that the drain technique must also be capable of routinely draining the glass melter whenever the melter reaches the end of its useful life. Typical electrical furnaces for melting glass are those disclosed in U.S. Pat. Nos. 3,524,206 and 3,852,509.
Two drain techniques which have been used by the art are bottom dump valves and evacuated canisters, both of which have encountered difficulties. An evacuated canister can be used to remove glass through the top of the melter. The number of evacuated canisters needed to empty a melter depends on the size of the canister. (Other methods of removing glass through the top of a melter have been proposed, but have not been demonstrated.) Bottom drain techniques can be conveniently divided into two classes, namely, dump valves and freeze valves.
A dump valve can be simply a piece of pipe in the floor of the melter that can be heated to initiate glass flow. When such valve is used, the entire contents in the melter is emptied into only one canister because glass flow cannot be stopped. The oversized dump canister that must be used complicates facility design and processing operation. For this reason, the dump valve is not suited for canyon use.
Dump valves are generally made from a suitable length of pipe, tubing, etc., which passes from the bottom of the melt pool through the glass melter floor. During normal operation the valve is air cooled in order to freeze the glass within it and prevent draining of the glass melter. To initiate glass flow, the air cooling is terminated and the valve is heated by one of several methods to a sufficiently high temperature to permit glass flow. The gravitational force acting on the glass in the melt pool then causes the glass to drain from the glass melter, at least in principle. In practice, considerable difficulty has been encountered in initiating flow through dump valves because of spinel deposits on the melter floor above the valve. Furthermore, even in a melter which is well designed and operated, some deposits are expected to accumulate on the melter floor due to corrosion of refractory sidewalls. Thus, it is uncertain whether flow through a dump valve can be initiated at the end of a projected two year melter lifetime if the valve has been inactive up to that time. By definition, dump valves are not intended to stop the flow of glass once it begins. Thus a suitable means of containing the entire contents of the glass pool must be available. Several standard size canisters are required to contain the entire contents of a typical glass melter. For such reason, dump valves are judged unsuitable for use in many glass melters.
Freeze valves are similar to dump valves except that, by definition, they are capable of stopping the glass flow once it begins, and in principle at least, are suitable for draining a glass melter. For any reasonable choice of freeze valve inside diameter (&gt;0.5 inch) and length (1 to 2 feet), it can easily be shown that for a typical glass pool depth of 2 to 3 feet it is not possible to terminate the glass flow once it begins by simply restoring the air cooling to the freeze valve. Indeed, even if water cooling were used, the required valve length to freeze the glass is unacceptable, and one runs into the additional complications of severe thermal cycling of the valve plus internal pressure surges due to the water flashing to steam within the freeze valve. Only if the glass flow through the freeze valve is very small does it become feasible to refreeze the glass and stop the flow. Thus, some means of controlling the applied pressure drop across the freeze valve is necessary, since such is the controlling variable once flow is initiated.
The art has proposed the method for draining glass out of the bottom of a melter using a jet-controlled freeze valve. Such valve is capable of stopping glass flow. When required, glass flow is stopped by applying air pressure at the mouth of the valve, and then freezing the glass in the valve body using cooling air. The air pressure at the exit is generated by air jets directed at the opening. Such type of valve is simple in design and has no moving parts.